I’m not familiar enough with SmugMug to WordPress integration to know if there’s a “better way” to do that cleanly, so won’t comment there.

I can’t speak to NextGEN Gallery or how it works.

If you don’t have programming resources in-house, I would look at off-the-shelf tools like W3 Total Cache, which as I understand it takes some of the pain about hosting resources in S3 – mirroring your content, and then handling pointing at those cloud files in a smart way.

My experience is on the DIY side of things — sorry I can’t be more helpful about your specific needs. I’d say generically that the cloud is great to the extent you have bandwidth you need to move elsewhere.